Non-cryogenic gas separation processes, especially adsorptive processes, are used to separate various components from a gaseous mixture, e.g., oxygen from air. Pressure swing adsorption (PSA) processes, including superatmospheric adsorption/desorption processes, subatmospheric vacuum swing adsorption (VSA) and transatmospheric vacuum pressure swing adsorption (VPSA) processes have been used for decades for air separation and are well known in the art.
Conventional PSA, VSA and VPSA processes employ positive displacement blowers for either fluid feed into or exhaustion from the adsorbent vessel. Large tonnage gas separation plants, including VPSA plants, require high flow of gas in and out of adsorption beds. Commercially available off-the-shelf blowers cannot supply the required flow of air into the plant, and custom-made blowers in this size range become prohibitively expensive. In addition, larger blowers generate higher pressure pulses in the plant that might damage the equipment and lead to louder noise levels.
In VPSA plants, blowers displace a large quantity of gas from inlet on the intake side to outlet on the discharge side at relatively constant volume via pockets between the lobes of each blower and the housing. The flow of gas in and out of a blower is not steady, rather it is a discrete action. Due to pressure differences between the gas pockets and piping, every time the rotor tips clear the housing, it causes pressure fluctuations. Such fluctuations create pressure pulsations. These pulsations are a function of blower size and speed. Within the piping and plant equipment, these pulsations manifest themselves as vibrations, which shake the piping and plant equipment and can result in severe damage. In ambient air, these pulsations manifest as noise, which can be extremely loud. As the demand for higher throughput out of these plants increases, the size of the plant and the size of the blowers in the plant increase. However, larger blower sizes and higher rotation speeds create higher pulsations, which could be detrimental to plant equipment such as the after-cooler, beds and pipes, and may also generate higher noise levels. Generally, the most damaging pulsations are generated at low frequency. The primary frequency of the pulsations generated by the blowers is the lowest frequency in the frequency spectrum, which makes it extremely challenging to cancel these pulsations.
To minimize the impact of the pulsations, gas separation plants may utilize blower inlet and/or discharge silencers. However, such silencers become prohibitively expensive for larger plants, and they decrease plant efficiency by inducing pressure drop in the flow. Even though these silencers can reduce the pulsations and noise, nonetheless, the pulsation problem is still present and needs to be eliminated by some other means.
Prior attempts to solve the pulsation and noise problem include the installation of a Helmholtz-type pulsation dampener, also known as a Helmholtz resonator (U.S. Pat. No. 5,957,664), cylindrical metal shell discharge silencers (U.S. Pat. Nos. 5,957,664 and 5,658,371), and underground type concrete silencers (U.S. Pat. No. 6,451,097). In particular, cylindrical metal shell type silencers are widely used in the industry, but they are not very effective for use with high amplitude and low frequency pulsations. In order to improve their effectiveness, it has been suggested that cylindrical metal shell type silencers be used in conjunction with a Helmholtz resonator (U.S. Pat. No. 5,957,664). However, these resonators are only effective in cancelling pulsations at certain frequencies. These silencing methods are based on reactive and absorptive sound cancellation principles. The biggest hurdle in designing a large gas separation plant is that it requires a much higher flow rate which can only be achieved either by using a single larger than commercially available blower or two smaller off-the-shelf blowers simultaneously. U.S. Pat. No. 5,656,068 disclosed a four-bed VPSA process, operated as two pairs of 2-bed systems, referred to as 2×2 cycle/system, to produce oxygen from air. Each pair of beds is operated 180° out of phase and the two pairs of beds are operated out of phase by one-half of a half-cycle. Two compressors (one Roots or positive displacement and one centrifugal) and two vacuum pumps (one Roots or positive displacement and one centrifugal) are used and one of the two compressors is periodically in the idle or vent mode. Although the use of two relatively small blowers instead of one large blower has been disclosed in U.S. Pat. No. 5,656,068, the active noise cancellation concept is not taught or used.
U.S. patent application Ser. No. 11/395,140 disclosed another approach that employs a silencer for reducing noise level to about 90 dB level at the discharge of the vacuum blower in large tonnage oxygen VPSA plants. The silencer comprises reactive chambers to attenuate low frequency pulsations and absorptive chambers to attenuate medium to high frequency noise. Outer and interior walls of the silencer are made of concrete. Unlike steel-shelled silencers, the concrete silencer will not vibrate or act as a noise source. The low frequency noise is cancelled by expanding and contracting flow in series of reactive chambers that have multiple openings in the dividing walls. The absorptive chambers enforce a serpentine flow, and their entire interior walls are covered with sound absorbing material to effectively cancel noise at high frequencies. However, this approach still requires the use of a large custom-made blower or multiple blowers to provide a sufficient feed gas supply.
Thus, there is a continuing need for low cost and reliable solutions to prevent pulsation damage and reduce noise levels while providing high flow of gas in and out of the plant in a cost effective manner.